Vector systems to deliver, integrate and express therapeutic genes in host cells are essential for gene therapy. In the present study, we investigated a novel vector system for integration and expression of a transgene. In this system, the transgene expression was driven by an endogenous RNA polymerase I (Pot I) promoter after being integrated into the ribosomal DNA (rDNA) locus. Human coagulation factor IX coding sequence (FIX), with an internal ribosome entry sites element at its leader region, was targeted into the 18S rDNA locus via homologous recombination. FIX protein expression, which was under the control of the endogenous Pot I promoter, was found to be similar to that of a moderate Pot II promoter. The average FIX expression level of the rDNA recombinants was additionally enhanced to that from a strong Pot II promoter as a result of elimination of position effects. Our data suggest the possibility of applying this system in gene therapy for hereditary diseases. (c) 2008 Elsevier Inc. All rights reserved.